typing: add stub functions for `cext/charencoding`
I'm not sure if it's better to have a separate file, and currently pytype
doesn't really know how to handle these, so it's no help in figuring that out.
Technically, these methods are part of the `mercurial.cext.parsers` module, so
put them into the existing stub until there's a reason to split it out.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true